Introducing ecodesign in silica sol-gel materials

Over the last decade, ecodesign has been introduced as a concept and a methodological framework to identify and improve sustainability in product development. In this context, the 12 principles of green chemistry provide suitable guidelines for the elaboration of molecules and materials in conditions that meet some ecodesign-related criteria. Sol-gel chemistry is an interesting domain to be examined in this perspective because it was early identified as an eco-friendly process compared to traditional routes to ceramics and glasses. Thus it is not surprising that many recent developments in sol-gel technology have, explicitly or not, addressed sustainability issues. In this review, we present an overview of these advances, focusing on the chemistry of silica. Starting from the typical reaction involving tetraethoxysilane hydrolysis and condensation in hydro-alcoholic media in the presence of inorganic catalysts, the current alternatives in terms of precursors, solvents, catalysts and activation sources are presented. As an example of hybrid materials, the synthesis of surfactant-based mesostructured silica is commented. Manufacturing methods to nanoproducts, including sol-gel technology are also discussed in terms of sustainability. Finally, the recyclability and degradation of sol-gel silica are briefly commented on. As a conclusion, some perspectives and current limitations for the development of a "greener" sol-gel chemistry are provided, extending the discussion to non-silica materials.